Put two gold coins on a table.
Both are one ounce. Both are 22 karat. Both were struck by a national government. Both have been traded worldwide for decades.
One is allowed in your IRA. One is not.
That is the American Gold Eagle and the South African Krugerrand, and the difference between them has nothing to do with the gold.
If that seems arbitrary, it is because the rule is not one rule. It is three separate tests, stacked, and a coin has to pass all of them.
Most people know about the first test. Almost nobody knows about the third, which is the one that rejects bars and coins that are provably pure.
Here is the whole structure.